Opacity Not 50%?

So sorry if this has been asked but I haven't really seen anything yet for this problem...

But anyway, I'll try to explain the best I can.

So I want to make a colour 50% opacity. Basically, I'm doing markings for a creature and they need to be able to be different opacities. I have no problem getting it to actually be exactly 50% when I put #000000 on #FFFFFF.

So that's all good. But now I want a marking that's a mix of 50% of #000000 and 100% #000000 on the same layer:

To get the lighter spots, I put the 'brush' to 50% opacity and didn't overlap, the colour being #000000. I would expect it does the same thing as just putting the layer itself at 50% opacity.

Looks fine at first, but when I pull the layer down to 50%, I would expect the dark spots to be 50% and the light spots to be pure #FFFFF white. But that's not the case. This is what happens:

So yeah...If my explanation is right, is there any way to fix this? Or am I doing something horribly wrong? I'm not brand new to gimp, but I am still reasonably new so bear with me.